ELEC1099 - Draft Teaching Plan by lm0N43u

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									                         MSc Computer Systems - Draft Teaching Plan
                                     Hardware Section


    The table below outlines a teaching plan for the MSc Computer Systems module. Please
                     note that this is for guidance only and WILL change.




Week        Lecture Queens         Lab Queens 1.01      Reading/Self Study
            0.17                   & 3.01/2

1           Introduction To        Introduction To      Web notes on information
            Number Systems         Lab                  representation

2           My First Computer

3           I/O Interrupts and
            DMA

4           Peripheral Devices

5           C Programming 1

6           C Programming 2




I can be contacted on email eg@dmu.ac.uk



Substantial web based learning material can be found on Ian Sexton’s Web-Site
www.cse.dmu.ac.uk/~sexton/WWWPages

They will be found under ELEC1099
The notes below were compiled from various sources, including Ian Sexton, Morteza Safazi,
and Kath Garnett. Most of the originals can be found on their respective websites, or on Ian
Sexton’s ELEC1099 WWWpages.
1 NUMBER REPRESENTATION



1.1 Decimal Numbers



In Europe we use the Latin alphabet to represent our diverse languages. However we
ceased using Latin Numbers a long time ago. You are unlikely to see the date that I wrote
these notes represented as -



       XXVII:VI:MMIII                   Latin Numerals



Instead we use Arabic numerals – 0,1,2,3,4,5,6,7,8,9 – to represent our number systems, so
our date looks like -



       27:06:2003             Arabic Numerals



Our number system is based on powers on 10 – for no other reason than we have 10 fingers
(and thumbs) to count up on. There are other number systems than ours; one lost
Polynesian culture only recognised three numbers, One, Two and Many. Even today there is
much debate as to whether or not Zero is a true number.



We represent our numbers as polynomials of powers of 10.



For example the number three-hundred-and-eight-six is written as 386. But this is just a
convention, what we are really writing is



       (3 x 100) + (8 x 10) + (6 x 1)



The first columns numbers are 1’s or 100

The second columns numbers are 10’s or 101

The third columns numbers are 100’s or 102

And so on
Numbers less than one are represented by –ve powers of 10. So 27.34 really means



       (2 * 101) + (7 * 100) + (3 * 10-1) + (4 * 10-2)



Where 10-1 is 1/10, 10-2 is 1/100 etc.



We represent –ve numbers by placing a – or minus symbol in front of the numbers. This
does not really make the numbers –ve, we just have a convention that tells us how to
interpret that symbol.



Our everyday number system is infinite. We can create almost any number we like from the
infinitely big to the infinitely small, they can positive or negative, they can even be imaginary.
Computers are not so clever, as we will see.



1.2 Binary Numbers



Computers are not as clever as we are, they are made up of electronic circuits known as
LOGIC GATES. These gates are either ON or OFF, and these states are used to represent
just two numbers - 0 or 1.



Computers deal with groups of binary digits called words. A word is a string of binary digits
that the computer regards as a single unit of data. An n-bit computer is one in which the
length of the most frequently used word is n and the main data paths are parallel n-bit paths.
The value of n depends on the computer's vintage, the purpose for which it was designed,
and its cost. It varies from 4 (in some simple microprocessors) to 64 (e.g. a Pentium) and
beyond.



We can build up a number system that is just as rich as our decimal numbers, with just two
numbers. The trick is to line them up in columns, and assign different values based on the
power of 2 to each column.



If we stick with 4 columns for now we can represent 16 different number –



       23      22      21       20       ‘Binade’

       ‘8’     ‘4’     ‘2’      ‘1’      Equivalent Decade
         0     0      0       0      0

         0     0      0       1      1

         0     0      1       0      2

         0     0      1       1      3

         0     1      0       0      4

         0     1      0       1      5

         0     1      1       0      6

         0     1      1       1      7

         1     0      0       0      8

         1     0      0       1      9

         1     0      1       0      10

         1     0      1       1      11

         1     1      0       0      12

         1     1      0       1      13

         1     1      1       0      14

         1     1      1       1      15



We can extend our binary number if we wish, but for the moment we will stick with 4 bits of
binary data – better known as a NIBBLE.



1.3 HEX Notation



Even though computers like binary, it is not easy for us to think in binary. Instead we use a
compromise notation known as HEXADECIMAL. HEX arithmetic is a number system based
on powers of 16 – therefore we need 16 characters to represent our numbers. For the first
10 numerals we use Arabic notation, for the additional 6 numerals we use Latin letters.



Binary         Decimal        HEX

0000           0              0

0001           1              1

0010           2              2

0011           3              3
0100           4              4

0101           5              5

0110           6              6

0111           7              7

1000           8              8

1001           9              9

1010           10             A

1011           11             B

1100           12             C

1101           13             D

1110           14             E

1111           15             F



We can now create larger numbers using multiple HEX digits. The first column will be
powers of 1, the 2nd column powers of 16 and so on. As each HEX digit replaces 4 binary
BITS, 4 HEX digits are the equivalent of 16 binary BITS.




For example the HEX number 3E7B is the same as –



Powers of 16          163     162    161     160

Equivalent decimal    4096    256    16      1

HEX Digits            3       E      7       B

Polynomial            (3*4096) + (14*256) + (7*16) + (11*1)



1.4 Octal Notation

Octal notation is a number system based on powers of 8. It is rarely, if ever used these days,
but you may come across it in old textbooks.

Binary         Decimal        OCTAL
000            0              0

001            1              1

010            2              2

011            3              3

100            4              4

101            5              5

110            6              6

111            7              7



1.5 Finite Number Sets


Our number system is infinite. However computers do not have infinite storage capacity, so
computer number systems are finite.



A memory location is defined by its’ ‘width’, which is another way of saying how many binary
bits can be stored in that memory location. Typical lengths are 1,4,8 & 16. These different
lengths are also known as a BIT (1), NIBBLE (4), BYTE (8) and WORD (16).



If a memory location is only a byte wide, then the biggest number that it can store is 255
Decimal, or FF HEX. A word memory can go up 65535 Decimal or FFFF HEX. What
happens if we overflow? Try it with a pocket calculator, enter the biggest number that you
think of, say 99999999999 exp 99, and add 1 to it; the probable result is that the calculator
displays ‘ERROR’ – why? Because computers are stupid. They have finite word lengths and
cannot ‘think’ of numbers greater than the ability of their memory to store them.



If you add 1 to a byte memory holding FF, it rolls around back to zero. We will see later how
computers handle overflows, by use of what is known as the CARRY FLAG.




1.6 NEGATIVE NUMBERS


In our infinite decimal system we use a ‘-‘ symbol to designate that a number is negative.
Computers have no mechanism to prefix a fixed bit length value with a ‘-‘ symbol; so how do
computers represent –ve numbers.
Consider our finite number system, using HEX notation. If we have 8 bits our number system
is a RING –



00 01 02 03 .... FD FE FF



When we add one to FF it wraps back to 00.



If these are considered to be +ve numbers only, the decimal equivalents to above sequence
are –



+0 +1 +2 .... +253 +254 +255



When we add 1 to 255 it goes back to zero. So if we subtract 1 from zero we get FF HEX



So 00 – 01 = FF



The HEX number FF therefore represents the negative number –1



Let us rearrange our RING



               FD FE FF 00 01 02 03

                Negative Positive



This notation is known as 2’s complement. The rule is that if the most significant binary bit is
a ‘1’ then the value is –ve, otherwise it is +ve.



If we have 8 binary bits we can use the 256 available patterns to represent an UNSIGNED
BYTE. The numeric range is 00 HEX to FF HEX, or 0 decimal to 255 decimal. Or we can
use them to represent a SIGNED BYTE. The numeric range is 80 HEX to 7F HEX, or –128
decimal to +127 decimal.

Here is a simple proof that it works.



00 = 0
FF = -1

FE = -2

FD = -3



What is –3 +4 = ?



FD + 04 = 01



1.8 Converting +ve Numbers to –ve Numbers



There is a simple technique that allows us to switch a +ve number to a –ve number, and
back again. Write out the number in binary, swap all the bits over, and then add 1.



Try it –



03 = 00000011                +3 decimal to binary

     11111100                swap the bits

     11111101                add 1

-3 = FD               convert to HEX



It works the other way around as well –



FD = 11111101                -3 in binary

      00000010               swap the bits

      00000011               add 1

      03              convert to HEX



1.9 Sign Extension



If we expand a signed byte, to a signed word, then we must maintain the status of the sign
bit. This is known as sign extension.
So the number +3 is 00000011 in binary. If it is sign extended to a 16-bit value it becomes
0000000000000011. The sign bit is extended into the new bit fields.



Now consider –3, which is 11111101 in binary. When it is sign extended to 16 bits it
becomes 111111111111111101.



1.10    Multiplication and Division



Start with the decimal number 37. To multiply it by 10 we simply add a zero to get 370. In
effect we have shifted our number to the LEFT, in order to multiply it by 10.



The same is true for binary, but in this instance shifting left multiplies by 2. So if we start with
0011 (3 decimal), and shift it left we get 0110 (decimal 6).



Shifting right divides a number by the base power. So 370 become 37, and 0110 becomes
0011.



1.11 Arithmetic Shift, Logical Shift and Rotate



Most computers support three different types of rotate instruction. This is a generalisation,
you must consult the computer’s Assembly Language Manual for the precise details of how
that computer works.



‘Arithmetic shifts’ are used for signed numbers, and maintain the status of the sign bit.
Consider the negative –6, which is 11111010. If we Arithmetically Shift Right we obtain

11111101 which is –3. Note that all the bits are shifted right, the least significant bit is lost,
and the most significant bit remains unchanged. By maintaining the sign bit we can
successfully divide signed –ve numbers by 2.



Arithmetic Shifts Left multiply by two. Some computers maintain the sign bit, others just allow
the bit 6 to shift into the bit 7 position (such as the 6805). As long as our result does not
exceed the available numeric range of –128 to +127, all will be OK
Logical Shifts simply shuffle all the bits left or right as required. If we shift left, bit 7 is lost,
and bit 0 is set to 0. If we shift right then bit 0 is lost and bit 7 becomes 0. These instructions
can be used to multiply & divide unsigned values, or just for bit manipulation.



Rotations consider the number as a ring, but with the addition of an extra bit called the
CARRY FLAG. If we rotate left then bit 7 goes onto the carry flag, bit 6 goes into bit 7 and so
on, and bit 0 is filled with the previous contents of the carry flag. Rotate right does it the other
way around.



1.12    What is the Carry Flag



You have seen the carry flag mentioned a few times. Its role is to capture the bit that
overflows (from bit 7) or underflows (from bit 0) as a result of any relevant arithmetic or
logical operation. So if we add together two numbers, with a result greater than FF HEX then
the carry flag is set to 1, else it is set to 0. It is in this case the ‘ninth’ bit. Similarly if we
subtract a big number from a little number it is set to 1, to indicate a ‘borrow’.



It is also used to capture overflows for shifts and rotates.



Some instructions will use the carry flag. For example the OP-CODE ADD will add together
two numbers, whereas the OP-CODE ADC will also add in the current value of the carry
flag. In this way we can perform arithmetic on numbers that exceed the word width of our
memory locations.



ADD16:                  LDA     LOWBYTE1                 ; Get low byte of 16-bit value 1

                ADD     LOWBYTE2                 ; Add low byte of 16-bit value 2

                STA     LOWRESULT                ; store the low byte of the result

                LDA     HIGHBYTE1                ; Get the high byte of 16 bit value 1

                ADC     HIGHBYTE2                ; Add high byte of 16 bit value 2, and carry

                                                 ; from low byte addition

                STA     HIGHRESULT2                      ; store the low byte of the result



Note that if our final answer is greater than FFFF HEX, then the carry flag is set.
All computers have a CARRY FLAG. They also have what is known as the ZERO FLAG,
which is set whenever the result of the last arithmetic or logical operation resulted in a zero.


1.13 Ian Sexton’s Notes Including Floating Point Notation
Information Representation

Introduction
The logic circuits of digital computers are binary, that is, they can at any one time be in one
of only two states. Thus, the only information that can be represented is that which is
adequately represented with only two values - say whether a switch is on or of, a person is
male or female.

The two states of a binary device are usually represented by the binary digits (bits) 0 and 1.

Computers deal with groups of binary digits called words. A word is a string of binary digits,
which the computer regards as a single unit of data. An n-bit computer is one in which the
length of the most frequently used word is n and the main data paths are parallel n-bit paths.
The value of n depends on the computer's vintage, the purpose for which it was designed,
and its cost. It varies from 4 (in some simple microprocessors) to 64 (e.g. a Pentium) and
beyond.

The representation of Data in a Word
For simplicity, consider a pattern of eight bits, say a word containing eight binary digits.
Within 8 bits the patterns which can be distinguished are:



00000000

00000001

00000010

etc.

11111110

11111111

It can be seen that there are 2n (256) different patterns. Therefore in an 8-bit word, 256
different values can be distinguished. Generally an n-bit word can be in one of 2n different
states. What information is being represented and what value a particular pattern has
depends on the context and how the data is coded in binary. Before discussing how
commonly needed information is represented for a computer and how it is coded, it is useful
to explain some notational conventions for binary words.

Notations and Conventions
The n-bit binary word is often written as:

an-1 an-2 ... a0, where ai is either 0 or 1

Thus considering the 8 bit word:
0 1 1 0 1 0 1 0, a7, a4, a2, and a0 are 0 and a6, a5, a3, a1 are 1

Alternatively, we can describe the individual bits by their 'bit number'. Starting from the right -
bit 0 and numbering each bit until bit 7 is reached.

In arithmetic contexts bit 0 is called the least significant bit (LSB) and bit 7 is called the most
significant bit (MSB).

Considering an 8-bit word, if the patterns in a binary word are used straightforwardly to
represent non-negative integers:

0 0 0 0 0 0 0 0 is 0

and

1 1 1 1 1 1 1 1 is 255

In general: X(10) = a727 + a626 + a525 +a424 + a323 + a222 +a121 +a020

Octal and Hexadecimal
Words written in binary notation are not convenient for human use; we are most adept with
decimal (denary) notation, which is also shorter. The most common shorthand ways of
writing binary information are called OCTAL and HEXADECIMAL. The conversions to and
from binary (if necessary) are much simpler than with denary.

Octal, or Base 8

A given word is split into 3-bit groups starting with bit 0; the least significant bit. For example:

01 101 010 Binary word

is

1 5 2 Octal word

To get the denary equivalent:

X(10) = 1x82 + 5x81 + 2x80

Hexadecimal
Hexadecimal is more popular because 8 bits can be represented by only two 'Hex' digits.
The notation is a trickier to master as alphabetic characters are used to represent the extra
digits. The table below shows the equivalent representations.

Denary Binary          Octal Hexadecimal



     0   00000     0       0

     1   00001     1       1

     2   00010     2       2

     3   00011     3       3

     4   00100     4       4
 5       00101     5    5

 6       00110     6    6

 7       00111     7    7

 8       01000     10    8

 9       01001     11    9

 10        01010   12     A

 11        01011   13     B

 12        01100   14     C

 13        01101   15     D

 14        01110   16     E

 15        01111   17     F

 16        10000   20    10

Bytes
A byte is the universally accepted name for a group of eight bits. A group of four bits is
sometimes called a 'nibble' being smaller than a byte!

Non-negative integers
Conceptually, the simplest interpretation of a bit pattern is as a non-negative integer and the
numerical value can be calculated as shown previously. In practice this coding is not used
very often but a particular use is in addressing memory locations in the computer where (in
simple terms) the amount of real memory a computer can address is determined by the
number of bits available on the Address Bus. For example, if the computer has a 16 bit
address bus then it can address 65536 (216) memory locations.

The range of values represented by an n-bit word is: 0 to 2n - 1

Integers
A range of consecutive integers, which run from negative, through zero, and then positive,
can be represented by a binary string. The most common convention for achieving this is
called 2's complement.

Remember that in an n-bit string we can represent at most 2n different values. Thus if half of
these values are negative then the maximum positive integer is reduced. Specifically, an 8-
bit word interpreted as 2's complement integers can represent numbers in the range:

-128 to +127

To understand how the 2's complement system works, imagine a 6-digit 'mileometer' on a
car, which is driven in reverse. The sequence of patterns on the meter as it passes through
zero are:

000003
000002

000001

000000

999999

999998

999997

It is natural to think of 999999 as -1 etc. Since 999999 can not represent both -1 and
+999999, a rule for interpreting the patterns must be used. If we were to say that:

000000 to 499999 represented positive integers

and

999999 to 500000 represented negative integers

we would have a decimal system exactly analogous to the 2's complement system for
representing integer values with binary digits.

Using an n-bit pattern the range is -2n-1 to 2n-1 -1 and all patterns which begin with a 1
represent negative values. This left most bit is called the sign bit. With 2's complement
notation the normal rules of addition apply. For example:

    2 000002 00000010

+

    -3 999997 11111101



= -1 999999 11111111

In order to find the additive inverse of any number i.e. the number to which it must be added
to yield an answer of zero, the following rule applies. To help you understand the jargon just
think of this technique as changing the sign - making a positive number negative, or a
negative number positive.

Change all 0's to 1's and all 1's to 0's (i.e. invert or complement the number) and add 1.

Thus, the additive inverse of

00110010 (50 in decimal) is 11001110 Explanation:

11001101 (by complementing)

11001110 (by adding 1)

In general, in adding two numbers we treat them as unsigned integers and disregard any
carry from the most significant bit. e.g.

 01010101       85
+ 11001100       -52

= 00100001       33

Consider the addition

 01000000          64

+ 01000010          66

= 10000010         130    ?????

Two positive numbers are added (the result should be 130 in decimal) but the answer
appears to be negative (bit 7 is a 1). What's wrong? The explanation introduces the concept
of an overflow. In an 8 bit pattern 127 is the largest positive integer that can be represented
using 2's complement notation - so 130 is outside the range; an overflow has occurred. In
essence an overflow occurs when a result is too 'big' for the available number of bits.

To represent a larger range of integers more than 8 bits are needed. If more than one word
is used to give a large enough range it is called multiple precision. Most commonly, two
words are used in double precision arithmetic.

Numbers with fractional parts - Floating Point
In the real world, the most useful arithmetic involves numbers that are not all integral, i.e.
have a fractional part. With suitable rules, these too can be represented using a binary
pattern.

Conceptually, the simplest method is to choose a place for the binary point. Bits to the left
are the whole number part and bits to the right are the fractional part. Thus, if the point is
taken to be between bit 4 and bit 3 in an 8-bit pattern: 01011101, it is interpreted as:

0x23 + 1x22 + 0x21 + 1x20 + 1x2-1 + 1x2-2 + 0x2-3 + 1x2-4

in a completely analogous manner to the digits in a decimal fraction. Since, in fixing the
position of the point, we are in effect, multiplying integers by a scaling factor (in this case 2-
4) positive and negative fractional numbers can be represented in twos complement form,
and the number of significant figures depends on the value of the number being represented.

Floating point
Floating point representation has two essential groups of bits, one of which, the mantissa,
contains a fixed-point binary fraction and the other, the exponent, contains an integer used
to calculate the scale factor. There are many ways in which these two parts can be coded,
as a 2's complement fraction and an integer, respectively, for example.

The number of bits allocated to each group depends on the computer, its word length and
use. The number of bits in the mantissa determines the number of significant figures, the
number in the exponent, the range of numbers that can be represented.

To indicate the principle, consider an 8-bit group in which the 3 left most bits are the
exponent and the 5 right most, the mantissa, both in 2's complement notation. Thus the
pattern:

10111011
has '101' (-3) for the exponent and '11011' (-0.15625) for the fraction which combine to give:

-0.15625 x 2-3

Generally, 2's complement notation is not used because the advantages it has for adding
integers, are not available when scaling factors are involved.

An unrealistic (but easy to follow) example of how 8 bits could be used to represent a
floating point number might be:

   1       011               1011

 sign     exponent             fraction

(negative) (-1 see below)   0.5 + 0.25 + 0.625 = 0.6875

Which is the same as saying: -0.6875/2 = -0.34375.

The three digits used for the exponent can have 8 possible values: 0 - 8, but if we subtract 4
from this number we can represent numbers both larger and smaller than the binary fraction.

More sensibly, a 32-bit representation might be used where 8 bits could be used for the
exponent, 23 bits for the binary fraction, and the remaining bit is used for the sign.



2 Flags


Computers store numbers in registers. If our register can only store a byte, then the largest
value that it can remember is 255. What happens if our result is greater than 255? Similarly
what happens if we subtract a large number from a small number? We need a mechanism to
indicate that the limited capability of our calculator store has resulted in an incorrect answer,
because there was either a CARRY or a BORROW.



This is indicated using a special 1-bit store called THE CARRY FLAG. If any arithmetic or
logical operation generates a ‘carry’, then the carry flag is SET, else it is RESET.



Examples of logical operations that can cause a carry are the SHIFT and ROTATE
operations that are discussed later.



All computers have a CARRY FLAG. The other flag that you will always find is known as the
ZERO FLAG. It is SET if the result of the last operation was a zero, else it is reset.



There are other types of FLAG that will come across, such as SIGN, HALF-CARRY,
PARITYand INTERRUPT. These are usually stored in a special register called the
CONDITION CODE REGISTER (CCR) or just known simply as THE FLAGS.
The half carry indicates that a carry has occurred between bits 3 & 4 of the accumulator. It
used for BCD arithmetic.



The sign flag indicates if the last arithmetic operation resulted in a –ve result (i.e. the sign bit
is set to a 1).



The interrupt flag indicates whether or interrupts are active. If it is set to a 1 then the Interrupt
Mask is SET, so interrupts are disabled



The parity flag is set if the parity of the accumulator is even.



3 My First Computer



The core of a computer is a component known as the Arithmetic & Logic Unit, or ALU. It is
able to perform a range of arithmetic and logical operations, such as ADD, SUBTRACT,
AND, OR etc.




The ALU shown here can perform one of 8 different operations. The Accumulator is always a
source of data, and the destination of the result. The other source of data (if required) will be
from any one of the other registers available in the computer.



How do we decide which instruction to perform? We could use a decoder to select the
operation.
We need 3 control lines to select one of 8 different arithmetic or logical operations, and 2
control lines to select the other source of data for that operation.




We can now construct some truth tables for our control lines:



 Control    Control     Control    Operation
 0          1           2

 0          0           0          ADD

 1          0           0          SUB

 0          1           0          INC

 1          1           0          DEC

 0          0           1          SHR

 1          0           1          SHL

 0          1           1          AND

 1          1           1          OR




Control    Control     Registe
3          4           r

0          0           0
1          0           1

0          1           2

1          1           3




So how do we ADD REGISTER 2 to the ACCUMULATOR?



To select ADD we set control lines 0,1 & 2 to 000, and control lines 3 & 4 to 01. When the
binary pattern 00001 is put onto the control lines then contents of register 2 will be added to
the accumulator – that is the way the circuit is designed.



3.1 Exercises


What bit pattern do we use to perform the following operations :



SUBTRACT REGISTER 1 FROM THE ACCUMULATOR

AND REGISTER 3 WITH THE ACCUMULATOR

OR REGISTER 0 WITH THE ACCUMULATOR



3.2 Programmes


We have now seen that if we apply a binary pattern to the control lines of our computer it
performs a known operation. The types of operation that we can perform are very simple –
the above sequence (exercise 8.1 and section 8) consisted of a sequence of 4 operations.



ADD REGISTER 2 TO THE ACCUMULATOR

SUBTRACT REGISTER 1 FROM THE ACCUMULATOR

AND REGISTER 3 WITH THE ACCUMULATOR

OR REGISTER 0 WITH THE ACCUMULATOR



This sequence is known as a PROGRAMME. A programme is a sequence of 1’s and 0’s
which when applied to the control lines of a computer will force to perform a known
sequence of operations. This is the link between the ‘programmer’ and the engineers that
designed the computer.
Binary patterns are recognised by our computer and are known as MACHINE CODE. They
are usually represented in HEX as opposed to binary. There is a limit to the number of
control lines that are found in a computer. A typical microcontroller would only have 8 control
lines, which means that there a maximum of 256 possible machine code instructions. These
are known collectively as the computer’s INSTRUCTION SET.



Human beings prefer words to numbers, so for each machine code there is a mnemonic
code that represents that instruction. For example our 4-line programme could be expressed
as -



ADD 2

SUB 1

AND 3

OR 0



This type of programming is known as ASSEMBLER CODE or ASSEMBLER LANGUAGE.
There is one assembler instruction for each available machine code. Note that each
instruction has two parts, one part is the actual operation itself, known as the OPCODE (e.g.
ADD), the other part is the memory location that contains the data that is operated upon, or
the OPERAND.



We will see later that there are different ways of selecting a unique memory location, known
as ADDRESSING MODES. For now we have defined only two types of ADDRESS, register
addressing and inherent addressing. Register Addressing selects a unique Register (in this
case 0,1,2 or 3). Inherent means that the OPCODE itself refers to a specific address, such
as INC and DEC, which always operate on the accumulator.



4 Memory


Computers require memory to store programmes and data. But what do we need to store in
the memory? The simplest partition of the memory is into 3 parts – CODE, DATA & STACK.
The code partition holds our programmes. The data partition holds our variables. The stack
partition is used to manage programme flow, parameter passing and interrupts.



Memory comes in a hierarchy, ranging from ‘slow, big and low-cost’ to ‘fast, small and high-
cost’.
The picture below show the simplest partition of our memory.




Code is usually placed at the bottom of memory, data dome next, then we have the ‘stack’.



4.1 How Do We Manage Programme Flow



First reconsider our 4-line assembler programme from earlier



ADD 2

SUB 1

AND 3

OR 0



Each of these commands will need to be stored in consecutive locations in our CODE
SEGMENT.
So how does our computer know where to find the next opcode? This is achieved using a
special register known as the INSTRUCTION POINTER (IP) or PROGRAMME COUNTER
(PC).



Let us review our picture – We need to add a DATA BUS that is used to move opcodes and
data from memory to the CPU, and results back into memory. We also need an ADDRESS
BUS that is used by the CPU to uniquely select a specific memory location, either to access
data or to obtain the next opcode.



This discussion assumes that we are using a computer with an 8-bit data bus and a 16-bit
address bus. A 16-bit address bud allows us access to 216 (65536) memory locations. An 8-
bit data bus means that our computer can support a maximum of 256 op-codes.




How does our computer manage the address bus? The programme counter is used to point
to the next op-code, so we cannot use that to point to data. We need another pointer to
select a data memory location; one method is to employ a special 16-bit register known as
the INDEX REGISTER.
Our instruction pointer is automatically set to 0000H when our computer powers up. So the
first op-code that will be executed is always at the bottom of memory. Our computer then
enters the ‘instruction-fetch-execute’ cycle. Each op-code is broken down into a series of
steps, which can be generalised as -



       1      FETCH THE OP-CODE

       2      FETCH THE OPERANDS

       3      EXECUTE THE OP-CODES

       4      STORE THE RESULT



So the op-code is not the lowest layer of our computer’s command structure, there is a lower
layer that breaks down each op-code into a series of ‘micro-instructions’ or ‘micro-code’.



When each op-code is ‘fetched’ from the code segment, the instruction pointer is
automatically incremented to point to the next available code segment memory location. This
may be an extension to the current op-code (e.g. data or an address) or it may the next op-
code. When our op-code has completed execution, the instruction pointer now ‘addresses’
the next op-code.



As our instruction pointer is a register, we can either make it increment as already
discussed, or we could reload it with a totally new address. Consider the next piece of code
–
In this instance, the op-code at location 2 loads the instruction pointer with the operand
0006. So our next instruction will be located at address 6 – it is the equivalent of the BASIC
instruction GOTO, and is the means by which we can implement a branch in our programme
flow.



4.2 Conditional Branching


Overwriting the instruction pointer using a JMP instruction. This type of branch is known as
UNCONDITIONAL, as it will always happen. The really powerful instructions are those that
combine the contents of the FLAGS with the branch instruction, these are the
CONDITIONAL BRANCH op-codes.



Consider just two of the flags, CARRY and ZERO. The carry flag is set if the last arithmetic
or logical operation generated a carry, the zero flag is set if the result was a zero.



All computers support the following four conditional instructions, in one form or another –



       JZ              Jump if zero

       JNZ             Jump if not zero

       JC              Jump if carry

       JNC             Jump if not carry



It is the existence of these instructions that gives a computer the ability to take decisions. All
high level constructs, such as IF THEN ELSE, DO WHILE, FOR etc. Are compiled down to
one of these types of op-code.
4.3 Subroutines



Another form of branching that we need to support are subroutine calls. At a low level you
would use commands such as –



CALL SUBROUTINE               ; RUN ‘FUNCTION’ USING A 16-BIT ABSOLUTE ADDRESS

JSR    SUBROUTINE             ; RUN ‘FUNCTION’ USING A 16-BIT ABSOLUTE ADDRESS

BSR    SUBROUTINE             ; RUN ‘FUNCTION’ USING A 8-BIT RELATIVE ADDRESS



The CPU saves the current contents of Instruction Pointer, and then loads it with the address
of the first line of the subroutine. The very last instruction of the subroutine will something
like RET or RTS. When this opcode is executed the contents of the instruction pointer are
restored, so that execution continues from the point that we called the subroutine from.



The ‘return address’ is temporarily stored in the STACK SEGMENT. Access to the stack is
via a special purpose pointer register known as the STACK POINTER. When a return
address is placed onto the stack it is said to be ‘pushed’ – the stack pointer automatically




decrements after each push operation, such that it points to the next free entry.



Other uses for the stack segment include passing parameters to subroutines, receiving
returned values from a subroutine, temporary storage of local variables and management of
interrupts.



4.4 Speed Of Access – Backup, Cache and Pipelines
At the time of writing the typical access time for MAIN MEMORY is 60ns. Our main memory
holds our code, data and stack segments. However it is only part of our memory hierarchy.
When we are not using programmes or data they can be stored on large, slow BACKUP
MEMORY devices such as hard disks, CD-ROMs, floppy disks and ZIP-Drives; we only
need to load them into our main memory when we want to use them.



Once our code and data are loaded into main memory, and we are allocated a stack
segment we can start executing the programme.



60ns sounds fast, but as today’s programmes consist of megabytes of code, and data
storage for images can be vast, it all adds up to a lot of time. To speed things up we use
another type of memory called THE CACHE. This is very high-speed memory that is located
close to our CPU, and runs at between 2ns to 25ns depending upon how it is organised.



Main memory is usually made from STATIC RAM, similar to the D Type latches discussed
earlier. Once data is written to them, it is stored for as long as power is applied to the
memory device. Cache is usually made from DYNAMIC RAM; each memory cell contains far
fewer active devices than dynamic ram memory, which is why they operate faster. However
the data that is stored on them decays (i.e. the data is lost), so they have top be regularly
REFRESHED, to restore the stored data.



Cache is used to speed up data segment access. If a data variable if used, then it is
probable that it will be used again very soon; so instead of putting it back into main memory
a copy is retained in the cache. In this way the CPU can access the most recently used data
very quickly. Cache locations are allocated dynamically, as they are required. When the
cache is full, the oldest data is put back into main memory freeing up a new location.



What about CODE? If we have just executed an op-code at location n, then the next
instruction is probably going to be at location n+1. Instead of waiting for instruction n to
finish, we might as well get the op-code at location n+1 whilst the CPU is executing
instruction n. This is known as PIPELINING. The execution part of the CPUs control logic
fetches more than just the next instruction, it fetches a batch of sequential instructions, ready
for execution. Some more advanced devices look out for branch operations, and fill up two
pipelines ready with both possible routes, so that it is always ready to deliver the next op-
code in the shortest time possible.



5 MEMORY MANAGEMENT


Memory Management Units are responsible for allocating available main memory space to
functions. This is a brief overview of how this task is achieved.
5.1 Virtual Memory



http://computer.howstuffworks.com/virtual-memory.htm



The above link gives a good description of virtual memory, with better pictures than mine. As
we have already discussed, modern programmes need a lot of memory – and if you start
executing a lot of programmes then you are liable to run out of main memory to keep them
in. Virtual memory overcomes this problem. The Virtual Memory Manager looks for available
RAM to store a new programme in. If there is not enough space then it looks for a block of
RAM that has not been used for some time (i.e. a programme that is ‘running’ but not
actually doing anything at that moment of time), and copies that programme’s RAM onto a
storage media such as a hard disk. This frees up main memory for the new task. When the
dormant programme starts up again, its’ ‘ram image’ is restored to main memory.



5. 2 Segmentation



This is a totally different process that is used to allocate the available main memory RAM to
a new task.



When we compile a programme it creates an executable that requires 3 memory segments,
CODE, DATA & STACK. These segments are referenced using pointers, such as the
INSTRUCTION POINTER, the INDEX REGISTER and the STACK POINTER. Let us
assume that these pointers are 16-bits wide; then each segment consists of only 64k bytes.



However our main memory is much larger than this, a MEGABYTE has 1048576 memory
locations. Segmentation is a technique that allows us to locate our programme segments
anywhere within the main memory.



Our programme address space is only 16-bits wide – which means that all memory pointers
have a range of 0000 to FFFF. However if we have a megabyte to play with, our real
memory space has a range of 000000 to FFFFFF, which is a 24-bit address.



In effect we have 256 ‘blocks’ of 64k each available. Our programme requires 3 of these, for
code, data and stack. The memory management unit allocates 3 free blocks for this new
task. The simplest way to do this is to have a set of segment pointers, each of which are 8-
bits, that are used to ‘offset’ our programmes blocks into the available main memory. This is
known as the EFFECTIVE ADDRESS, as it is the actual location in main memory where will
find our information.
An effective address is obtained by combining the programme's pointer information with the
segmentation offset.



   EFFECTIVE ADDRESS = POINTER (lower 16-bits) + SEGMENT OFFSET (upper 8-bits)



With the above method CODE, DATA and STACK can never overlap.



Another technique, as used in PCs, is to allow an overlap between the pointer address and
the segment offset. In this case the pointer gives us the lower 16-bits, and the segment gives
us bits 4 through to 20. This technique provides a more efficient use of main memory, as
unused gaps are removed – however it can cause problems as poorly written programmes
may allow data to be overwritten with code, and visa-versa.



6 ADDRESSING MODES


So far we have only talked about using memory pointers, such as the Stack Pointer,
Instruction Pointer and Index Register. The Index Register allows us to point to any data
location of our choice, but it requires us to preload the Index register with an address. There
are more direct forms of addressing data that can be used.



REGISTER

IMMEDIATE

DIRECT

INDIRECT

INDEXED



6.1 Register Addressing



All CPU's contain some internal registers, which act like local fast data stores. For example
an 8086 type CPU contains 4 'general purpose' registers called AX,BX,CX and DX.
Programmes can access these registers as an integral part of their op-codes,



e.g. MOV AX,BX will move the contents of register BX into register AX.
You are unlikely to ever make use of REGISTER ADDRESSING unless you are prepared to
abandon your compiler and write your code in assembler. Some C compilers allow you
force a variable to be stored in a REGISTER by a slight modification to the variable
declaration. e.g. register unsigned int name - will force the compiler to try to reserve a
register for the variable 'name' to be stored in a register. You are not advised to use this
facility.



6.2 Immediate Addressing



IMMEDIATE ADDRESSING means that the number that we are loading is part of the
instruction itself, and is not found in the data memory.



e.g. MOV AX,#334 - will load the AX register with the number 344.



The following two lines perform the same operation in C



int my_variable ;       // declare an integer Variable called 'my_variable'

my_variable = 344 ;     // store the number 344 into my_variable



With immediate addressing the data is fixed in the code, and is therefore a CONSTANT. In
the example given above the number 344 is the constant.



6.3 Direct Addressing



DIRECT ADDRESSING means that the opcode includes the address of the variable that we
wish to load. e.g. MOV AX,my_variable - the accumulator AX is loaded with the contents of
my_variable.



Or in C



int my_variable ; // declare an integer variable called my_variable

int another_variable ; // declare another variable

my_variable = another_variable ; // whatever was stored in 'another_variable' is
                            // copied into 'my_variable'
You may have now realised that variable names are really just alternative ways of using an
absolute memory address. When the compiler 'compiles' our source code, it first allocates a
unique memory address for each and every variable that is declared. The machine code
accesses memory using numeric addresses, but we access memory by using the names
that we declare in our source programmes.



With direct addressing our data address is fixed, but the data value itself can vary.




6.4 Indirect & Indexed Addressing



INDIRECT ADDRESSING means that the address of the variable that we wish to address is
not included in the op-code at all, instead we are given the address of a different variable
that contains the address the actual variable that we wish to access.



At a low level this can be achieved by means of either an INDEX or a POINTER register.
These are special registers in the architecture of the computer that are specifically designed
for use as indirect registers.



In an 8086 one these registers is called BP or base pointer. If we load BP with a number,
then that number can then be used as the address of a variable



e.g.

MOV BP,#344            ; Load the base pointer with the number 344

MOV AX,[BP]                   ; Load the AX register with the contents of the memory location

                       ; pointed to by BP



When an internal register is used as a pointer it is known as INDEXED ADDRESSING.



Alternatively we can use another memory location as our data pointer.



e.g.
MOV 178,#344              ; Load memory location 178 with the number 344

MOV AX,[178]              ; Load the AX register with the contents of the memory location

                          ; pointed to by the contents of memory location 178



When an external memory location is used as a pointer it is known as INDIRECT
ADDRESSING.



In C this is achieved as follows -



unsigned int my_variable ;       // declare a variable for us to play with

unsigned int *pointer ;          // declare a pointer to an unsigned integer



So we have a variable to use, and we have a pointer as well. Remember that when we start
our programme the pointer DOES NOT HAVE A VALUE, as yet it does not know what to
point at.



I will now introduce another C concept, the 'address' symbol, which is a '&'. The & operator
returns the address of the variable that it is applied to.



my_variable = 344 ;              // load 344 into my_variable

pointer = &my_variable ;         // load pointer with the address of my_variable



We can now either directly address my_variable using its' name 'my_variable', or we can
indirectly address the identical memory location using the pointer. When a pointer is used we
use the * symbol to indicate that we wish to access the variable that the pointer 'points to'.



IMMEDIATE ADDRESSING -

my_variable = 344 ; // the source data is the constant 344



DIRECT ADDRESSING -

another_variable = 344 ;

my_variable = another_variable ;         // the source data is the contents of address
                                         // 'another_variable'
INDIRECT ADDRESSING

pointer = &another_variable ;

another _variable = 344 ;

my_variable = *another_variable ; // the source data is the contents of the
                      // memory location pointed to by the contents of pointer




7 Input and Output I/O


How do we get information into and out of a computer? There are two main techniques,
Serial I/O and Parallel I/O.



Parallel is the easiest to consider first. Data is transferred between the CPU and memory
using the DATA BUS. A Data Bus is no more than 8, 16 or 32 wires in parallel. A PARALLEL
PORT is an electrical interface to this bus, instead of the data being presented to a memory
location it is instead presented to a series of contacts on a connector. For example a printer
port on a PC is a parallel interface – 8-bits of data from the data bus are put onto the pins of
the printer connector, another wire called the strobe is asserted to inform the printer that
another character is available for it to read. Parallel ports can be uni-directional (as just
described), or bi-directional which means that data can be transferred in both directions
using the same connections.



Parallel connections are very fast, because each bit of data has its’ own wire to move along.
They are not practical for long distances, not least because of the cost of the cabling.



For long distances we use SERIAL I/O. Examples of serial I/O are a PC’s COM PORTS,
USB, FireWire and Ethernet. Today we will only consider Serial I/O.



A PC’s COM port conforms to the CCITT V24 standard, better known as RS232. Instead of
sending out all data bits at the same time, we clock them out individually one after the other
over a single wire – thus the name Serial I/O. Each character is ENCAPSULATED between
a START BIT (0) and a STOP BIT (1).



7.1 Direct Memory Access DMA
Serial and parallel I/O both require direct intervention by the CPU, by means of a suitable
programme (often called Drivers in PCs). An alternative technique is to use DMA.



DMA is a technique that allows data to be moved into and out of memory without any
intervention by the CPU. It can be achieved in a number of different ways. An external DMA
Controller can take over the CPUs data bus, effectively locking the CPU out of memory.
Whilst the CPU is locked out the DMA controller can perform the memory transfer.



Another technique is known a ‘cycle stealing’ – the CPU only accesses memory when it
needs to (i.e. to get the next op-code or to retrieve data), whilst it is not using the memory
the DMA controller is allowed access.


8 INTERRUPTS


In an ideal world the occurrence of events, and the actions that need to be taken, can be planned for and
scheduled into a nice clean multi-tasking multi-use operating system. However we all know that that is not
the real world.



External events can, and will, occur whenever they want to, and the computer must react to them within a
specified time. For example we may have an electronic control unit (ECU) for a car, that measures road
speed and updates the driver's speedometer. To achieve this the time of arrival of pulses from the road
speed sensor must be logged almost immediately (depending upon the accuracy required) whatever the
processor may be doing. So the processor must suspend its’ current operation, and pass control
immediately over to the road speed sensor function. The processor is INTERRUPTED by the external
event, which takes over control of the processor and all its’ resources.



8.1 Examples of Interrupts



In the simplest of systems there will normally be at least two possible interrupt sources. One will be an
EXTERNAL INTERRUPT, and the other will be a TIMER INTERRUPT.



8.1.1 External Interrupts



As its’ name implies an external interrupt, is a hard wired signal taken to the processor from an external
source. This could be, for example, a pulsed input from an external frequency source, or a crash sensor on
a car. This signal is usually digital, and the interrupt is triggered when that signal changes state. External
interrupts can be either LEVEL SENSITIVE or EDGE SENSITIVE.
Edge sensitive signals detect a change of state, e.g. high to low or low to high. Level sensitive signals are
sensitive to a high or a low level. Edge sensitivity is more commonly used as level sensitivity can cause
multiple interrupts to occur from the same source.



External interrupts are also classified as MASKABLE or NON MASKABLE. A maskable interrupt is one that
can be enabled and disabled by software, which means that the processor is capable of accidentally
switching it off should there be a software error.



Maskable interrupts are usually used for processing expected external signals, such as limit switches,
frequency signals, and peripheral requests for attention. In all these examples it is of advantage to the
programmer to be able to disable the interrupt source, e.g. there is no point allowing an interrupt to occur
during an initialisation or idle phase.



But what if the interrupt is an emergency warning signal, such as a crash sensor, or watchdog time-out. Fail
safe procedures are degraded if it is possible for faulty software to switch off the alarm signals, therefore
NON MASKABLE INTERRUPTS must be used for such signals.



TYPES OF EXTERNAL INTERRUPT



RISING EDGE SENSITIVE

FALLING EDGE SENSITIVE

HIGH LEVEL SENSITIVE

LOW LEVEL SENSITIVE



Any of which can be MASKABLE or NON MASKABLE



8.1.2 Timing Interrupts



Timing interrupts are generated internally by the processor itself. An independent digital divider circuit uses
the processor clock as timing source. This divider is can be set up by the programmer to divide the main
clock source down to give a slower timebase. Simple processors usually offer only a binary division (i.e. by
128 or 256 etc.), more expensive devices (i.e. £2 or more) offer a division register, typically 16 bits wide,
but sometimes only 8 bits. When this register counts down to zero an interrupt is issued to the processor.



So for example if we are using a 4 MHz clock, and we need a 10 ms time base the divider register would
be loaded with 40000. 4 MHz / 40000 = 100 Hz that is 10 ms.



One thing to look out for is whether or not the timer automatically reloads the division ratio. Some
processors do this (e.g. INTEL 8051 FAMILY, but only 8 bit reload), most DO NOT, therefore the timer will
need to be reloaded on every occurrence of the interrupt.
Another trap that is waiting for you is the fact that sometimes the divider COUNTS UP and not down,
issuing the interrupt when the register rolls over from FFFF to 0000. In this instance you will need to
preload the divider with the complement of the number that you wish to divide by, i.e. 65536 - DIVISION
RATIO.



TYPES OF TIMING INTERRUPT



UP COUNTER                Interrupt when register changes from full (FFFFH or FFH) to zero

DOWN COUNTER              Interrupt when register changes from 1 to zero



TYPES OF DIVISION

BINARY DIVISION           Can only divide by 2/4/8/16/32/64/128 etc

8 BIT DIVISION            Counter is only 8 bits wide, so can divide by 1 to 255

16 BIT DIVISION           Counter is 16 bits wide, so can divide by 1 to 65535

AUTOMATIC RELOAD          Counter automatically reloads a fixed number on interrupt




8.1.3 Serial Interrupts



Most reasonable processors include at least one, and possible two serial communications devices on the
chip. One will be an asynchronous port, for use with an RS232 or RS485 driver, and the other will be some
form IIC/SPI driver.



As such links are often quite slow with respect to the processor, an interrupt is an essential requirement.
When data is received by a serial device it will issue a RECEIVER FULL interrupt, and when it is ready to
transmit data it will issue a TRANSMITTER EMPTY interrupt.



Normal software practice is to implement a cyclic buffer under interrupt control for both the receiver and
transmitter sections. The background process can then examine the size of the RECEIVE BUFFER to see
if any data has arrived, or put a character into the SEND BUFFER for interrupt processing.



The main dangers with this approach are as follows -



RECEIVER - the RECEIVE BUFFER must be polled, and emptied, by the background faster than data
arrives, else the buffer will overrun.
TRANSMITTER - when there is no data left to send the TRANSMITTER EMPTY INTERRUPT must be
disabled, otherwise it will hang up the processor forever.



8.1.4 Watchdog Interrupts



Watchdogs are used to provide system integrity. They are timing devices, often located externally to the
processor, but sometimes they are included on the chip itself. They act like an alarm clock, as long as they
are RESET by the processor at regular intervals they never time-out. If the processor dies, or gets lost in
some obscure section of code that has been poorly designed, then the timer does times out, and it issues
an interrupt.



Internal devices, as can be found on the PIC family, the Motorola 68HC11 family and some advanced 8051
type devices, use an internal interrupt or force a complete processor reset. External watchdog devices
(available from many manufacturers - such as DALLAS and MAXIM) provide a digital output that the
designer can either use to RESET the processor or cause an EXTERNAL interrupt.



The simplest method of using such a device is to set the ‘time-out’ to be longer than the complete
programme cycle; then at the end of the cycle a signal is sent to the watchdog to RESET its’ timer back to
zero. As long as the programme is operating correctly then the watchdog will never time out. This method
however cannot be used for asynchronous programmes that use interrupt functions. It is necessary to
ensure that each function independently ‘kicks’ the watchdog. This can be achieved by using flags. For
example, an interrupt function can set a flag indicating that it has occurred, and the background routine can
examine this flag, and only if it set will it then ‘kick’ the watchdog. In this way we have proven that both
functions have been executed within the desired time.



8.1.5 Power Fail Interrupts



It would be helpful if the processor had advanced warning of an impending power failure, in order that it can
gracefully close down all current operations and protect its’ memory. This is achieved by the use of power
fail interrupts, which are to be found in a number of advanced microcontrollers. They work by monitoring
the voltage level on the chip. If it falls below a minimum threshold it issues the interrupt call before the
power falls too far for the device to operate - you then have a small amount of time do something to make
everything safe and secure.



8.1.6 Software Interrupts (Traps)



A total separate group of interrupts may also exist, that are designed to ‘trap’ illegal operations. For
example sections of memory may be designated as protected, perhaps because it is designated as the
stack, or we may try to perform a hardware divide by zero. Illegal operations such as these can cause an
interrupt, so that some form of housekeeping software can be invoked to tidy up the mess.



8.2 Vectors & Saving the Processor Status
If we have so many different types of interrupt how can the processor distinguish between them ?



This is achieved by an INTERRUPT VECTOR TABLE. The vector table is located at a fixed location in the
code space, typically at the bottom of code space (location 0000 to 00FF) or at the top of the code space
(FF00 to FFFF). Each interrupt has a fixed location in the vector table for its’ own use, into which must be
stored the starting address of its’ unique interrupt function.



When any interrupt occurs, the processor stops running whatever function is currently operating, and
executes a SUBROUTINE CALL to the address stored in the vector. Therefore the minimal interrupt code
must be a single RETURN instruction, so control will then return back to the interrupted function -



However to ensure that an interrupt cannot itself be interrupted, it is usual for all other interrupts to be
disabled whilst any interrupt function is being executed. As the interrupt may wish to call subroutines, a
special type of return is used that signals to the processor that interrupts can now be reenabled. In the
INTEL family of devices this opcode is RETI, MOTOROLA uses the opcode RTI for the same purpose.



e.g. For an Intel 8051



code address      0000     reset vector

                  0003     external interrupt     (say 0100) >>>>>>>>>>>>>>>>>>>>>>

                  000B     timer 0 overflow interrupt                                     |

                  0013     external interrupt 1                                           |

                  001B     timer 1 overflow                                               |

                  0023     serial interrupt                                               |

                                                                                          |
                                                                                 |
                                                                        |

                                                                                          |

                                                                                          |

                                                                                          |

                                                                                          |

                  0100     Start of EXT0 interrupt function    <<<<<<<<<<<<<<<<<<<|

                           RETI




We are also using the processors internal resources, and data that was resident in its’ registers must be
saved before the interrupt code is executed, otherwise they may destroyed. This is achieved by pushing
the contents of any registers used by the interrupt function onto the stack, and restoring them before
executing the return operation. e.g.



                 0100     Start of EXT0 interrupt function

                          PUSH     A                                  ; SAVE REGISTER A

                          PUSH     B                                  ; SAVE REGISTER B

                          PUSH     DPH                                ; SAVE DATA POINTER

                          PUSH     DPL                                ; SAVE DATA POINTER




                          INTERRUPT CODE CAN NOW USE REGISTERS A/B/DPH AND DPL




                          POP      DPL

                          POP      DPH

                          POP      B

                          POP      A

                          RETI



All this pushing and popping takes time, and interrupts must be quick, so only save the contents of
registers that are used by the interrupt routine.



What about our processor status flags, these are not registers but represent the state of the machine at the
time the interrupt occurred. For example what was the state of the ZERO and CARRY flags, if they are not
correctly restore then any subsequent conditional branch operation in the interrupted routine will fail. These
flags must be saved as well, and special instructions are always present to allow this to be done.



                 0100     Start of EXT0 interrupt function

                          PUSH     PSW                       ; SAVE PROCESSOR STATUS

                          PUSH     A                         ; SAVE REGISTER A

                          PUSH     B                         ; SAVE REGISTER B

                          PUSH     DPH                       ; SAVE DATA POINTER

                          PUSH     DPL                       ; SAVE DATA POINTER
                        INTERRUPT CODE CAN NOW USE REGISTERS A/B/DPH AND DPL




                        POP      DPL

                        POP      DPH

                        POP      B

                        POP      A

                        POP      PSW

                        RETI



So to summarise an interrupt must cause the following to happen



* THE CURRENT PROGRAMME COUNTER ADDRESS MUST BE PUSHED ONTO THE STACK

* THE START OF THE INTERRUPT FUNCTION MUST BE READ FROM THE INTERRUPT                  VECTOR
TABLE

* THE PROCESSOR STATUS WORD MUST BE SAVED ON THE STACK

* THE CONTENTS OF ALL REGISTERS USED BY THE INTERRUPT MUST BE SAVED ON THE STACK

* EXECUTE THE INTERRUPT CODE

* RESTORE THE SAVED REGISTERS

* RESTORE THE PROCESSOR STATUS WORD

* REENABLE INTERRUPTS EXPLICITLY, OR IMPLICITLY WITH A SPECIAL RETURN

* RETURN TO THE CALLING FUNCTION




9 My PC


So how does all the above relate to the PC on your desk?



9.1 The CPU and MOTHERBOARD


Most of the above hardware will be found on the ’motherboard’ of your PC. The Central
Processing Unit (e.g. a Pentium) contains the ALU, the instruction fetch and execute logic,
the pipeline, the flags, the registers, the timing circuitry, the stack pointer, the programme
counter and the data pointers.
Close to the CPU will be found the Cache, which is fast dynamic RAM. Also on the
motherboard you will find the main memory, the address bus and data bus, and probably
some of the I/O devices.



Now some terminology. So far we have referred to the address bus and the data bus; there
is also a control bus that includes control lines such as ‘read from memory’ or ‘write to
memory’. Collectively these are called the xxx BUS, where xxx may be ISA or PCI or VME or
whatever BUS standard this particular computer manufacturer has chosen to use.



9.2 Memory


Backup memory is provided for by hard disk drives, floppy disk drives and CD-ROMs. All
three of these devices have file structures that are created and managed by the computer’s
Operating System or OS. Other types of backup media, such a Zip drives, do not use the OS
file structure.



9.3 I/O


Serial communication can be obtained via a COM port, or a USB port.



COM ports use the RS232/V24 asynchronous communication standard. PC manufacturers
are trying to get rid of them, but they are extensively used in telecommunications and will not
become obsolete despite some ridiculous claims by the mass-market PC manufacturers. A
typical use for a COM port is to connect to a dial-up MODEM – they are also used to connect
to ‘Packet Switch Networks’ such as an X-25 PAD (Packet Assembler and Dis-assembler),
and the mobile phone network via GSM MODEMS.



USB ports are a high speed serial standard, now widely used to connect a range of
peripheral devices to a PC. Currently, the USB Specification, Revision 2.0, covers three
speeds 480 Mbps, 12 Mbps, and 1.5 Mbps. The term "Hi-Speed USB" refers to just the 480
Mbps portion of the USB Specification. We now use the term "USB" to refer to the 12Mbps
and 1.5Mbps speeds.

See http://www.intel.com/technology/usb/faq.htm for more information.



You make also come across the concept of ‘networking’. A network is a cabling system that
connects a group of computers together into as ‘Network’. This allows users at on computer
to access resources and files on other computers that are connected to that network. It is the
job of the OS to allow users to apply ‘attributes’ to the files and directory structures in order
to allow or bar other users from accessing their data.
ETHERNET is an example of networking standard that allows messages to be moved from
one computer to another; it is not a complete networking solution, as it does not offer any
form a file management. You may have also heard of Novell – this is a network standard that
can overlay Ethernet, and provides the ‘networking’ layer that is lacking in Ethernet. There
are other standards available.



PCMCIA slots are very popular on portable PCs. These are in effect a collapsed BUS, that
allows developers to design slim-line peripherals that can by plugged into the computer to
provide a new facility, such as extra memory or a MODEM.



All PC’s have a parallel port, which is usually used as a printer connection.



9.4 Human Machine Interfaces


As a minimum these will be a screen, keyboard and a mouse.


Screens come in two main technologies, CRTs (cathode ray tubes) and LCDs (liquid crystal
displays). CRTs are big and bulky, and use the same technology as a TV – three different
coloured spot sources (Red Green and Blue) are scanned across the screen. The more lines
that your CRT can scan the better the vertical resolution. TVs have 625 lines, but in practice
some of these are blank, and as the image is ‘interlaced’ the effective resolution is halved to
about 300 lines. As each spot is scanned across your screen its’ intensity is changed. The
speed at which this happens determines the horizontal resolution.



So a screens overall resolution is defined in terms of (horizontal spot changes per line) x
(total number of lines). A basic VGA screen offer 640 x 480 resolution, which means that
there are 480 lines, each with 640 horizontal dots. Each dot is known as a Picture Cell, or
Pixel for short.



LCDs are made of individual dots, each of which can be switched on or off. You may be able
to see them if you look closely at an LCD or Plasma screen.



Keyboards are matrix-scanned devices, which converts each key depression into an 8-bit
character (not ASCII) or a 16-bit character pair. This information is transmitted to the PC
using a serial standard called Keyboard Wedge. Other devices that use the keyboard wedge
standard are barcode scanners, infrared sensors and RFID (radio frequency ID) sensors.



Mice can use a variety of techniques, USB, serial or keyboard wedge.
10 C Programming Language


This section is not a complete introduction to C; such information can be found from a variety
of sources at DMU. It is intended only to relate some C language concepts to material that
we have already covered.



10.1 Data Types


At the beginning of these notes we referred to different lengths of binary bits that are used to
represent numbers. The names that we used were BIT, NIBBLE, BYTE & WORD. Numbers
can unsigned or signed. Later on we discovered how floating point numbers are
represented.



In the C programming language most of these methods of representing numbers can be
found. They are known as DATA TYPES.



An 8-bit variable is known as a char. It can be signed or unsigned. We can create a char
variable for use in our programme by declaring a variable. The programme line -



char    x;



will create an 8-bit variable that is called ‘x’. In this case it will be a signed variable, as that is
the default. So x can have a numeric value in the range –128 to +127.



If instead we wanted an unsigned variable it would be declared as -



unsigned char y ;



This creates an 8-bit variable that is called ‘y’. It will be an unsigned variable. So y can have
a numeric value in the range 0 to 255.



16-bit variables are declared using the data type int.



int     x;
Will create a 16 bit signed variable, with a numeric range of –32268 to + 32267.



unsigned int x ;



Will create a 16 bit signed variable, with a numeric range of 0 to 65535.



C also supports a 32-bit variable type, which used to be known as a long int, but is now just
known as long.



long        x;



Will create a 32-bit signed variable, with a numeric range of –2147483648 to +214748367.



unsigned long x ;



Will create a 16-bit signed variable, with a numeric range of 0 to 4294967296.



C also supports two floating point variables types.



float x ;



Will create a 32-bit floating point variable that has an 8-bit exponent and a 24-bit mantissa.



double x ;



Will create a 64-bit floating point variable that has a 16-bit exponent and a 48-bit mantissa.
10.2 Operators



C supports are range of mathematical and logical operators. Most of which are self
explanatory. The maths operators are



+-/*



For Addition Subtraction Division and Multiplication. The usual BODMAS rules apply, which
means that multiplication & division take precedence over addition and subtraction; but it is
best to use brackets to ensure that you get the correct mathematical operation.



The logical operators are



&|^!



For And Or Exclusive-Or and Not



The logical operators should not be confused with the interrogative operators that are used
to build conditional statements



&& || ~



These interrogative operations will be explained later.




Numerical Representation Exercises
1 Convert the decimal number 98 into Binary, Octal and HEX



Method

First convert the number in BINARY

An 8-bit Binary polynomial has columns with the following values -

         128   64     32     16      8      4      2       1

Which are derived from the exponents of the power of 2.

20 = 1

21 = 2

22 = 4

23 = 8

24 = 16

25 = 32

26 = 64

27 = 128

So now we build up the binary number column by column



As 98 < 128 the 128th column is zero

         128   64     32     16      8      4      2       1

         0     ?      ?      ?       ?      ?      ?       ?



Now we try the 64th column

As 98 > 64 we set the 64th column to a 1 and subtract 64 from our number

         128   64     32     16      8      4      2       1

         0     1      ?      ?       ?      ?      ?       ?

98 - 64 = 34



Now we try the 32nd column

As 34 > 32 we set the 32nd column to a 1 and subtract 32 from our number

         128   64     32     16      8      4      2       1
       0      1       1      ?      ?      ?      ?   ?

34 - 32 = 2



Now we try the 16th column

As 2 > 16 we set the 16th column to a 0

       128    64      32     16     8      4      2   1

       0      1       1      0      ?      ?      ?   ?



Now we try the 8th column

As 2 > 8 we set the 8th column to a 0

       128    64      32     16     8      4      2   1

       0      1       1      0      0      ?      ?   ?



Now we try the 4th column

As 2 > 4 we set the 4th column to a 0

       128    64      32     16     8      4      2   1

       0      1       1      0      0      0      ?   ?



Now we try the 2nd column

As 2 = 2 we set the 2nd column to a 1

       128    64      32     16     8      4      2   1

       0      1       1      0      0      0      1   ?

2 -2 = 0



As our remainder is 0 the last column must be 0

       128    64      32     16     8      4      2   1

       0      1       1      0      0      0      1   0



So 98 DECIMAL = 01100010 BINARY
Which is another way of saying that

98 = 128x0 + 64x1 + 32x1 + 16x0 + 8x0 + 4x0 +2x1 + 1x0



Now we can convert our binary number into HEX and OCTAL equivalents

To create HEX equivalents we group the binary digit into groups of 4 bits

01100010 = [0110] [0010]

Our HEX number system has 16 different digits -



HEX           BINARY         DECIMAL

0             0000           0

1             0001           1

2             0010           2

3             0011           3

4             0100           4

5             0101           5

6             0110           6

7             0111           7

8             1000           8

9             1001           9

A             1010           10

B             1011           11

C             1100           12

D             1101           13

E             1110           14

F             1111           15



So using our table above

[0110] = 6

[0010] = 2
So our HEX value is 62H      NOTE THE H SUFFIX USED TO DENOTE THAT THIS IS
                             A HEX      NUMBER



Which is another way of saying that 98 = 6x16 + 2x1



Our OCTAL number system has 8 different digits -



OCTAL                BINARY         DECIMAL

0             000            0

1             001            1

2             010            2

3             011            3

4             100            4

5             101            5

6             110            6

7             111            7



To create our OCTAL number we group together three bits.

So using our table above

[001] = 1

[100] = 4

[010] = 2



So our OCTAL value is 142O          NOTE THE O SUFFIX USED TO DENOTE THAT THIS
IS                                  AN OCTAL NUMBER



Which is another way of saying that 98 = 1*64 + 4*8 + 2*1



2 Convert the following decimal numbers into BINARY HEX AND OCTAL



65
247

17

33

160



3 Convert the following numbers into decimal



A7H

0AH

80H

220H



220O

777O

101O



01011100B

11000000B

01010101B



4 Multiply the following in binary, showing all your working



100101B     x       101B

101000B    X        1010B



5 Divide the following in binary, showing all your working



1001000B        /    100B

1010001B        /     10B
6 Perform the following multiplications in HEX



3H x 2H

8H x 2H

8H x 3H

10H x 10H



7 Perform the following Divisions in HEX



1476H / 10H

80H / 4H

C0H / 4H

100H / 8H



8 Convert the following decimal numbers into 8-bit 2’s complement binary notation



-1

-10

-127

-33



NUMERICAL REPRESENTATION KEY TUTORIAL QUESTIONS

1. Draw up a table that shows the relationship between the first 16 numbers, counting from

 zero, expressed as decimal, hexadecimal, octal and binary.

2. How many binary bits are there in the following number ‘types’

     NIBBLE
     BYTE
     WORD


3. What is the largest decimal number that can be represented by 8 binary bits?

4. What is the largest decimal number that can be represented by 16 binary bits?
5. What is the largest decimal number that can be represented by 10 binary bits?

6. What is the decimal equivalent of 1k?

7. What is the approximate decimal equivalent of the maximum number represented by 20

  binary bits?

8. Convert the following decimal numbers into binary

  45
 129
  53
 195



9. Convert the following binary numbers into HEX

  00101101
  10000001
  00110101
  11000011


10. Convert the following decimal numbers into HEX

    65
   127
   245
     9

  (Hint – convert the decimal into binary, and then convert the binary into HEX)

11. Convert the following signed 8 bit –ve binary numbers into their –ve equivalents

   00000001
   00110011
   01010101
   01111110


12. What is the binary representation of the following –ve numbers, express your answer

   using 8 bits.

   -1
   -128
   -17
   -53

   (Hint – first convert the +ve value into binary, then use the 2’s complement conversion
   technique to create the –ve binary equivalent number)




13. Convert the following HEX numbers into OCTAL

   17
   31
    9
   11


13. Convert the following HEX numbers into OCTAL

   1A
   3C
   09
   17

   (hint – convert the HEX into binary, and then convert the binary into OCTAL)

14. Complete the following HEX sums, assuming that we are using signed 8-bit arithmetic

    80 – 0A
    15 + 95
    05 – 07
   1A + C7


15. What is the numeric range of 8 binary bits if we are using unsigned notation?

16. What is the numeric range of 8 binary bits if we are using signed notation?

17. What is the numeric range of 16 binary bits if we are using unsigned notation?

18. What is the numeric range of 16 binary bits if we are using unsigned notation?

19. Approximately how many memory locations can be addressed by a 24-bit address bus?

20. Complement the following binary numbers

   01010101
   11110000
   10101111
   00001000


21. Negate the following binary numbers

   01010101
   11110000
  10101111
   00001000


22. What is the format of a floating-point number? What is meant by the terms MANTISSA

   and EXPONENT?

								
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